North Korea said it may test missiles on a weekly basis, and warned of “all-out war” if the US takes military action.

What is the US doing about Iran?

In a statement on Wednesday, Mr Tillerson said a review, which he had announced in a letter to Congress a day earlier, would look at the whole US policy towards Iran – taking in not only Tehran’s compliance with the nuclear deal but also its actions in the Middle East.

He accused the country of “alarming and ongoing provocations that export terror and violence, destabilising more than one country at a time”.

“Iran is the world’s leading state sponsor of terrorism and is responsible for intensifying multiple conflicts and undermining US interests in countries such as Syria, Yemen, Iraq, and Lebanon, and continuing to support attacks against Israel.”

As part of a long list of charges, he criticised Iran’s involvement in the Syrian conflict and its support for President Bashar al-Assad.

The secretary of state earlier acknowledged the Iranians had met the terms of the 2015 nuclear deal. But he said its “nuclear ambitions” remained “a grave risk to international peace and security”.

Iranian Foreign Minister Javad Zarif reacted angrily on Twitter, saying “worn-out US accusations” could not “mask its admission of Iran’s compliance” with the deal’s requirements. He called on the US to change course and fulfil its own commitments.

President Donald Trump has previously vowed to dismantle the nuclear deal but has not specified what he wants to do.

What is the Iran nuclear deal?

The landmark 2015 agreement saw crippling sanctions on Iran lifted. It was secured after the International Atomic Energy Agency (IAEA) certified that Tehran had restricted its sensitive nuclear activities.

Iran says its nuclear programme is entirely peaceful and that it will continue missile development.

One of the key requirements of the deal is the reconfiguration of Iran’s Arak nuclear reactor so it does not produce weapons-grade plutonium usable in a nuclear bomb.

On Thursday, Beijing said companies in China and Iran would be working together on the redesign to ensure these terms would be met.

How has the US position changed?

Analysis by BBC State Department correspondent Barbara Plett Usher

In announcing a broad review of Iran policy the Trump administration has not jettisoned the nuclear deal.

But Rex Tillerson has come pretty close to saying the agreement is not worth keeping, even though he’s had to admit it’s working.

This week the secretary of state informed Congress that Tehran is keeping its side of the bargain to restrict its nuclear programme in exchange for the lifting of sanctions, which he’s required to confirm every 90 days.

In spoken remarks, though, he talked only of Iran’s bad behaviour and linked that to the future of the deal – a message that will resonate far more on Capitol Hill and to which it was probably aimed.

Former President Barack Obama would have agreed with all the charges: that Iran is a state sponsor of terrorism, that it supports proxies which undermine US interests in the region, that it’s hostile to Israel and that its ballistic missile tests challenge UN Security Council prohibitions.

But Mr Obama kept those issues separate from the nuclear agreement, which would have been impossible to achieve without that narrow focus.

Mr Tillerson, on the other hand, called this a mistaken approach and said the review would take a comprehensive look at all of the threats posed by Iran.